1. Field of the Invention
The present invention relates to ion mobility spectrometers (IMS) and electron capture detectors (ECD) comprising ion drift tubes comprising reaction regions, drift regions and ion collectors, wherein analog memory cells with exposed floating gates are used for the movement and detection of ions. The floating gates of the analog memory cells are programmed to suitable potentials to develop electric fields along the axis of the reaction region and the drift region for the movement of the ions generated in the reaction region into the drift region. The exposed floating gates of the analog memory cells in the drift region are also used for collecting the ions and the charges of the collected ions are determined by the changes that take place in the potentials of the floating gates of the analog memory cells.
2. Description of Prior Art
Ion mobility spectrometers (IMS) and electron capture detectors (ECD) for gas chromatographs and mass spectrometers have been developed for the detection and analysis of organic vapors, contaminants in the atmosphere and trace amounts of combat gases among other things. A typical drift tube used in these instruments comprises a reaction region for generating ions, a drift region for separating the ions and one or more ion collectors for collecting and detecting the charges of the ions. Sometimes a shutter grid is positioned between the reaction region and drift region which controls the flow of ions into the drift region. A carrier gas together with a sample gas introduced into the drift tube through the inlet is ionized by a radioactive source in the reaction region. Other methods such as photoionization have also been proposed or used for generating the ions.
The ions formed through these processes move to the ion collector(s) under the influence of an electric field which is applied along the axis of the ion drift tube. The mixture of ions is separated into individual ions before arrival at the collectors by the collision with drift gas molecules which normally flow in the opposite direction of ion flow and each ion shows unique mobility due to their mass, size, and charge. The separated ions produce an electric current upon contact with the collectors. The individual ion mobilities and the resulting currents of the ions arriving at the different collectors provide a basis for the identification of the chemical species and amounts originally introduced into the reaction region.
The specificity and sensitivity of prior art drift tubes is largely limited by the means used for measuring the small currents generated by the collected ions. Conventional electrometer circuits are only capable of measuring currents down to a few femtoamps (10.sup.-15 A) at best even when the collectors have a large surface area. Therefore there have been many attempts to develop charge detectors for ion mobility spectrometers, gas chromatographs and mass spectrometers of new design having better specificity and sensitivity, smaller size, easier manufacturing method and lower cost.